In recent years, transmission speeds of signal lights in optical communication systems have been speeded up from 10 Gb/s (gigabits/second) to 40 Gb/s, for example. With this speed-up of optical transmission, a modulation format for signal light has been shifted from intensity modulation to phase modulation. In comparison with the intensity modulation, the phase modulation has merits such that: receiver amplitude becomes twice by applying differential receiver so that receiver sensitivity is improved by 3 dB; since signal light intensity is fixed, an influence of a noise accompanied with an intensity change is small so that tolerance to a nonlinear effect during fiber transmission is high; and the like.
In an optical transmitter corresponding to the phase modulation format of 40 Gb/s or the like, a circuit configuration applying a distributed constant type amplifier is typically used in order to realize a drive circuit which generates a drive signal of high-speed and large amplitude. In such a drive circuit, it is necessary to consider changes in duty ratio of the drive signal and output amplitude thereof, which occur due to variations of power source voltage, temperature and the like. Incidentally, the duty ratio of the drive signal may be treated as a cross-point level in an eye pattern waveform of the drive signal, and therefore, in this case, a change in the cross-point level needs to be considered.
As a conventional technology relating to a driving of an optical modulator and a control thereof, there has been known a technology which superposes a low frequency signal on a drive signal corresponding to input data, and based on a detection result of a low-frequency signal component contained in a signal light (modulation signal) output from the optical modulator, controls a direct current bias voltage for regulating an operation point of the optical modulator (refer to Japanese Patent No. 2642499).
Further, for an optical transmission circuit corresponding to a carrier suppressed return to zero (CS-RZ) modulation format or the like, there has been proposed a technology which measures an optical spectrum of an output light from an optical modulator and controls a bias voltage so that power density of a carrier spectrum component becomes minimum (refer to Japanese Laid-open Patent Publication No. 2003-234703). This control technology focuses on characteristics of the CS-RZ modulation format in that the carrier spectrum component is suppressed at an optimum bias point, but appears when a bias point is deviated, to thereby optimize the bias point of the optical modulator.
However, in the driving of the optical modulator and the control thereof according to the above conventional technology, there is a problem in that the changes in duty ratio (or cross-point level) of the drive signal and output amplitude thereof in the above described phase modulation format may not be controlled. Namely, in the conventional control, the direct current bias voltage for regulating the operation point of the optical modulator is to be controlled, but the duty ratio (cross-point level) of the drive signal and the output amplitude thereof which follow the input data at a high bit rate are not to be controlled. Further, change characteristics in the optical spectrum due to variations in the power source voltage, the temperature and the like in the intensity modulation format, such as the CS-RZ or the like, are different from those in the phase modulation format, and therefore, it is difficult to control the drive signal corresponding to the above variations in the phase modulation format only by monitoring the carrier spectrum component. Namely, for the case where the optical modulator corresponding to the phase modulation format is driven by the drive circuit which uses the distributed constant type amplifier, it is necessary to realize a control technology capable of optimizing not only the direct current bias voltage, but also the duty ratio (or cross-point level) and output amplitude of the drive signal which is changed at a high speed in accordance with the input data.
Incidentally, as described later in detail, it is possible to deal with the control of the output amplitude of the drive signal in the phase modulation format, by applying the conventional technology which superposes the low-frequency signal on the drive signal. However, it is difficult to optimize the duty ratio (or cross-point level) of the drive signal only by such an application, and accordingly, it is necessary to establish a new control method adaptable to the phase modulation format.